Boron carbide (also referred to here as B4C) is the third hardest material next to diamond and cubic boron nitride. Combined with its low theoretical density (2.52 g/cm3), B4C is the premier material for personal armor-typically in the form of front and back flat plates which are bonded to a polymer backing and used as ballistic inserts in flack jackets. B4C is also used for nuclear shielding applications because of boron's high neutron absorption cross-section. In addition, B4C is used in particulate form as an abrasive, and as a nozzle material for slurry pumping and grit blasting because of its excellent abrasion resistance.
Effective ballistic armor materials must have very high hardness combined with high fracture toughness. When a high-velocity projectile makes contact with the surface of a ballistic material such as B4C, a compressive shock wave extends hemispherically from the point of contact, generating tensile, tangential stresses which cause radial cracks that emanate from the point of contact. These tangential stresses tear open cracks, preferentially at the site of pores and fissures. Therefore, ballistic performance of B4C improves with decreasing porosity, i.e. with increasing fired relative density.
Achieving near-theoretical density has required gang-hot pressing (stacked parts under pressure). Hot pressing does not allow for the cost effective fabrication of complex shapes. For example, the fabrication of form-fitting body armor parts would require machining after the hot pressing process, which is expensive and technically difficult.
Complex shapes (including form-fitting parts) are possible with pressureless sintering. According to the prior art, agents such as carbon, SiC, Al2O3, TiB2, AlF3 and W2B5 have been used as sintering agents in pressureless sintering to increase the sintered density. However, second phases due to the agents often have deleterious effects on the mechanical behavior of B4C.
The best known sintering agent for B4C is carbon. According to one prior art method, phenolic resin is used as a source of carbon. The carbon from the phenolic resin is distributed around the B4C particles, and also serves as a pressing agent.
Relative densities up to 98% have been obtained using carbon as a sintering agent. Carbon, when used as a sintering agent in pressureless sintering, however, promotes undesirable secondary phases and materials such as graphite which adversely affect the mechanical properties of the B4C.
Pressureless sintering of B4C without sintering agents has been difficult. Schwetz et al. in U.S. Pat. No. 4,195,066 cites to studies in which B4C has been pressureless sintered at near melting temperatures. However, the resulting material suffered in one study from low relative densities, and in the other study from poor mechanical properties compared to materials produced by hot pressing. In addition, Schwetz et al. noted that because the process required reaching close to the melting temperature of B4C it impaired the dimensional stability of the specimens.
In U.S. patent application Ser. No. 10/867,442, assigned to the assignee of the present invention, the subject matter of which is incorporated by reference, a method is disclosed whereby boron carbide can be sintered through pressureless sintering without intentional addition of sintering agents. While pressureless sintering according to Ser. No. 10/867,442 can result in high relative densities, graphite may still be present in the sintered boron carbide body. The graphite so present is native meaning that its presence is not due to the addition of any intentionally added (non-native) sintering agents added to the B4C powder to promote sintering. Rather, the native graphite may appear because of the volatilization of B4C, or it may appear due to the presence of excess carbon (carbon in excess of the required stoichiometric B4C) in the B4C powder.
It is commonly-known by those knowledgeable in the field that the hardness of B4C decreases precipitously with increasing content of free carbon (graphite) second phase. Because harness is an important property of B4C it is desirable to remove as much of the graphite as possible.
Furthermore, in a method according to prior art high isostatic pressing was applied in order to increase the relative density of the pressureless sintered B4C bodies. It is desirable to increase the relative density of the pressureless sintered boron carbide bodies to improve the mechanical properties thereof.